1. Field of the Invention
The present invention relates to a thermally assisted magnetic head having an asymmetric plasmon antenna, a head gimbal assembly and a hard disk drive, and to a method for manufacturing a thermally assisted magnetic head having an asymmetric plasmon antenna.
2. Related Background Art
Thin-film magnetic heads must deliver ever greater performance to cope with higher recording densities in hard disk drives. Widely used thin-film magnetic heads include composite thin-film magnetic heads having a multilayer structure comprising, for instance, a magnetic sensing element such as a magnetoresistive (MR) effect element and a magnetic recording element such as an electromagnetic coil element. These elements write and read data signals to/from a magnetic disk, which is a magnetic recording medium.
The magnetic recording medium is normally a so-called discontinuous medium having a structure in which magnetic microparticles are aggregated, each magnetic microparticle constituting a single domain structure. One recording bit comprises a plurality of magnetic microparticles. In order to enhance recording density, therefore, the magnetic microparticles must be made smaller, and the irregularities at the boundaries between recording bits must be reduced. Reducing the size of magnetic microparticles, however, is problematic in that reduction in particle volume is accompanied by a drop in magnetization thermal stability.
The factor KUV/kBT is an indicator of magnetization thermal stability. KU is the magnetic anisotropy energy of the magnetic microparticles, V is the volume of one magnetic microparticle, kB is the Boltzmann constant, and T is the absolute temperature. Making the magnetic microparticles smaller implies reducing their volume V. In turn, this makes KUV/kBT smaller, thereby impairing thermal stability. An approach for addressing this problem is increasing KU commensurately, but doing so results in a larger coercitivity of the recording medium. In contrast, the strength of the write magnetic field afforded by the magnetic head is largely determined by the saturation flux density of the soft magnetic material that makes up the magnetic poles in the head. Thus, writing may become impossible when the coercitivity exceeds a tolerance that is determined on the basis of the limits of the strength of the write magnetic field.
Proposed methods for solving the problem of magnetization thermal stability include using a magnetic material having a large KU, and so-called thermally assisted magnetic recording, in which writing is carried out by lowering coercitivity through heating of the recording medium immediately before application a write magnetic field. Thermally assisted magnetic recording can be broadly classified into magnetic dominant recording and optical dominant recording. In magnetic dominant recording, writing is governed by an electromagnetic coil element, and the radiation diameter of light is larger than the track width (recording width). In optical dominant recording, on the other hand, writing is governed by a light-radiating section, and the radiation diameter of light is approximately equal to the track width (recording width). That is, magnetic field determines the spatial resolution in magnetic dominant recording, whereas light determines the spatial resolution in optical dominant recording.
Japanese Patent Application Laid-open Nos. 2001-255254 and 2003-114184 disclose thermally-assisted magnetic heads in which an electroconductive plate-shaped plasmon antenna is disposed on a medium-facing surface, and in which near-field light is generated by irradiating light onto the plasmon antenna from an opposite side of the magnetic recording medium. The plasmon antenna is formed in such a manner that a corner at one end of the plasmon antenna is located close to a main magnetic pole. Near-field light is generated mainly around this corner. The stronger the intensity of near-field light that is generated at a corner that is located close to the main magnetic pole, the higher the temperature at which the recording medium can be heated. This is advantageous in terms of increasing the recording density of the hard disk drive.
When near-field light of strong enough intensity so as to carry out thermally assisted magnetic recording is generated at a corner on one end of the above conventional plasmon antennas, near-field light of some intensity is generated also at corners on other ends of the plasmon antenna. When the intensity of near-field light generated at corners on other ends is strong, that near-field light may heat up areas around the recording region of the recording medium, thereby giving rise to problems such as side-erasing. Meanwhile, when the intensity of the excitation light that is irradiated onto the plasmon antenna is weakened in order to weaken in turn the intensity of near-field light generated at corners on other ends, the intensity of near-field light generated at the corner on one end of the plasmon antenna becomes weaker as well, which makes it difficult to achieve high recording density. In order to achieve high recording density while suppressing problems such as side erasing, there must be reinforced the relative intensity of near-field light generated at a corner on one end of the plasmon antenna, namely the relative intensity of near-field light generated at a corner on one end with respect to the intensity of near-field light generated at corners on other ends. However, it has not been possible thus far to sufficiently increase the relative intensity of near-field light generated at a corner on one end of conventional plasmon antennas.